A large proportion of the emissions from an internal combustion engine are formed before the catalytic converter has reached its operating temperature during a cold start. Therefore, to be able to comply with the increasingly stringent emission regulations for motor vehicles, it is desirable for a catalytic converter to be heated as quickly as possible.
In this respect, EP 1 108 873 B1 discloses a method for heating a catalytic converter which is arranged in the exhaust-gas path of an internal combustion engine with direct injection and spark ignition, with a first fuel injection taking place in the compression stroke before top dead center of the respective piston, the air/fuel mix which is formed being spark-ignited and a second, postinjection of fuel being carried out between approx. 25° and 100° after top dead center. Furthermore, the postinjection brings the initially still lean air/fuel mix up to stoichiometric or rich (enriched) ratios.
The inventors herein have recognized an improved method for quickly heating a catalytic converter. Accordingly, a method for heating a catalytic converter which is arranged in the exhaust-gas path of an internal combustion engine with direct injection and spark ignition, includes a first fuel injection taking place before top dead center; and a second fuel injection taking place in the combustion stroke after top dead center, wherein the second fuel injection begins prior to ignition of the fuel.
The method according to the invention can be used in particular to heat a catalytic converter which is arranged in the exhaust-gas path of an internal combustion engine with direct injection and spark ignition. Heating of this type is required, for example, during an engine cold start in order for the operating temperature of the catalytic converter to be reached as quickly as possible and therefore for the removal of harmful emissions from the exhaust gases to be effected as quickly as possible. In the method, the injection of the fuel into the respective combustion chambers of the internal combustion engine is divided into at least two individual injections, a first fuel injection taking place before the piston reaches top dead center at the end of the compression stroke. Furthermore, a second fuel injection takes place after the piston has moved beyond top dead center and is therefore in the combustion stroke (also known as the expansion or working stroke). According to the invention, the second fuel injection is to begin prior to ignition of the fuel. An advantage of this invention is that it allows the combustion of the fuel to be very strongly delayed, with a high utilization of the fuel, which in turn correspondingly increases the exhaust gas temperature and therefore heats the catalytic converter quickly. The second fuel injection, which has commenced prior to the spark ignition, during the combustion stroke generates a region within the fuel chamber which has an enriched air/fuel mix which is readily ignited and causes the combustion to rapidly progress into the remaining regions of the combustion chamber, in which the typically lean air/fuel mix from the first injection operation is located. It has been found that this effect advantageously occurs substantially independently of the design of the combustion chamber. For example, the cylinder head may be designed in a conventional way, and the top side of the piston may be flat, concavely recessed or elevated in the shape of a roof. Accordingly, the fuel can be transported from the fuel injector to the spark plug by one or a combination of the methods known for this purpose, i.e. with guidance by the walls of the combustion chamber (“wall-guided”), by turbulence within the combustion chamber (“air-guided”) or by the shape and direction of the fuel jet (“jet-guided”).
The first fuel injection preferably takes place as early as during the intake stroke of the associated piston, i.e. when the piston is moving from the (previous) top dead center to the bottom dead center and in the process draws in fresh air through the open intake valve. The incoming flow of air leads to the formation of considerable turbulence, which brings about a good distribution of the fuel from the first injection.
In the first injection, the quantity of fuel is preferably such that it leads to the formation of a lean air/fuel mix in the associated combustion chamber. With a lean mix of this type, the risk of spontaneous ignition is ruled out or minimized. The first fuel injection may preferably comprise between 50% and 80% of the total quantity of fuel injected in the corresponding working stroke of the respective piston.
In a preferred embodiment of the method, the ignition of the fuel takes place between approx. 10° and 40°, preferably between approx. 25° and 30° after top dead center of the piston, in the combustion stroke. The result of such a delayed ignition is that the exhaust gases from the combustion are at a high temperature, the heat quantity which is contained in the exhaust gas being approximately proportional to the extent to which the ignition is delayed.
In another advantageous embodiment of the method, the second fuel injection begins between 0° and approx. 40°, preferably between 0° and approx. 25°, after top dead center of the corresponding piston in the combustion stroke. The second injection may optionally also end again within the angle ranges mentioned.
According to another embodiment, the second fuel injection extends beyond the instant of ignition of the air/fuel mix.
It is advantageous for the composition of the air/fuel mix, as seen over a working stroke of a piston, to be lean on average (in terms of time and space, i.e. within the combustion chamber), in order to achieve optimum utilization of fuel.
The invention also relates to an internal combustion engine with a catalytic converter arranged in the exhaust-gas path. The internal combustion engine has at least one cylinder, which includes a piston, a fuel injector for the direct injection of fuel, and an ignition device for the spark ignition of the fuel. A controller is connected to the fuel injector and the ignition device and, if appropriate, further actuators and/or suitable sensors, in order to control the instants and quantitative proportions of fuel injections as well as the instants of ignition. The controller can control the fuel injector in such a way that a first fuel injection takes place before top dead center of the associated piston and a second fuel injection begins in the combustion stroke after top dead center and before ignition of the fuel.
In a preferred embodiment of the internal combustion engine, the system further includes at least one pressure sensor coupled to one of its combustion chambers for measuring the combustion pressure in this combustion chamber. In this case, the engine controller is preferably designed to terminate the second fuel injection when the measured pressure in the combustion chamber exceeds a predetermined threshold value. Said threshold value in this case typically corresponds to the fuel injection pressure which the fuel injector provides or the maximum fuel injection pressure which it can provide, minus a safety margin to account for any tolerances.
The above advantages and other advantages, and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings, and from the claims.